MIGRATION VERSUS INVERSION IN ELECTROMAGNETIC IMAGING TECHNIQUE

One of the most challenging problems in electromagnetic (EM) geophysic al methods is developing fast and stable methods of imaging inhomogene ous underground structures using EM data. In our previous publications we developed a novel approach to this problem, using EM migration. In this paper we demonstrate that there is a very close connection betwe en the method of EM migration and the solution of the conventional EM inverse problem. Actually, we show that migration is an approximate in version. It realizes the first iteration in the inversion algorithm, b ased on the minimization of the residual field energy flow through the profile of observations. This new theoretical result opens a way for formulating a new imaging condition. We compare this new imaging condi tion with the traditional one, obtained for simplified geoelectrical m odels of the subsurface structures. This result also leads to the cons truction of a solution of the inverse EM problem, based on iterative E M migration in the frequency domain, and gradient (or conjugate gradie nt) search for the optimal geoelectrical model. However, the authors h ave found that in the framework of this method, even the first iterati on, based on the migration of the residual field, generates a reasonab le geoelectrical image of the subsurface structure.